- Server configurations (best practice/install guide for the OS platform using a front-end server and sql server). Not only the installation and the 'simple' settings that are available in the install checklist but IIS application pool architecture, best configuration for multiple processes/much users at the same time in the application, sql-server tuning, how to handle 'old' table data, sql server architecture and so on, how to apply multiple catalog, best way to use full cpu/ram.

- Information how to get the full platform benifits (what is the best way to set data to a table with navigation, why use/not use referenced actions (with text/Rec/RL parameters) instead of quering to the tables directly, settings when the platforms apply the benifits (production mode, queries in screen action), prefent sql injection/url manupilation, BTP settings. Maybe together with above topic: how to set seo url/server side caching.

- More information/practice how to use integration studio

I think it would be great if there were some 'example' eSpaces by subject (like best way to set table paging or how to handle queries with more joins/data, example of some highly used extensions (fileSystem/xml records), webservice adoption and some more :).

1) Architecture - how to setup espaces the right way (datamodel, businesslogic, front-end, backend) to think of the future and have many developers

2) How to setup a common espace with login, exception pages etc including 1 theme that will be used over multiple espaces.
And how to deal with the menu in this case (each espace will have it's own espace)

3) data merging from production to acceptance en development. How to keep the base-data correct.

1) Architecture - how to setup espaces the right way (datamodel, businesslogic, front-end, backend) to think of the future and have many developers

Great idea!

2) How to setup a common espace with login, exception pages etc including 1 theme that will be used over multiple espaces. And how to deal with the menu in this case (each espace will have it's own espace)

eSpaces having its own individual menu options?

3) data merging from production to acceptance en development. How to keep the base-data correct.

Refactoring? How data is ported over from one eSpace to another without losing its data?

Theming is definitly an important topic to discuss, 6.0 Platform has a new approach to Themes/CSS. Excellent idea Robert.
- How to build one StyleProvider taking in consideration the 6.0 new Themes/CSS approach.

Tricks and tips
-Intelliwarp (Tumbs up Robert, once again!)
- Multi-Language
- BPT (Do's and Don'ts and best practices)

Extension Development
Extension applicability (when/where/why do you need to build one?), Do's and Don'ts, best practices, and especially in-depth insight of the platform's classes in the various namespaces (OutSystems.*).

I believe most of us learned through trial and error, as there isn't that much information on the subject. :)